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.TH FTOK 3C "Feb 17, 2023"
.SH NAME
ftok \- generate an IPC key
.SH SYNOPSIS
.nf
#include <sys/ipc.h>

\fBkey_t\fR \fBftok\fR(\fBconst char *\fR\fIpath\fR, \fBint\fR \fIid\fR);
.fi

.SH DESCRIPTION
The \fBftok()\fR function returns a key based on \fIpath\fR and \fIid\fR that
is usable in subsequent calls to \fBmsgget\fR(2), \fBsemget\fR(2) and
\fBshmget\fR(2). The \fIpath\fR argument must be the pathname of an existing
file that the process is able to \fBstat\fR(2).
.sp
.LP
The \fBftok()\fR function will return the same key value for all paths that
name the same file, when called with the same \fIid\fR value, and will return
different key values when called with different \fIid\fR values.
.sp
.LP
If the file named by \fIpath\fR is removed while still referred to by a key, a
call to \fBftok()\fR with the same \fIpath\fR and \fIid\fR returns an error. If
the same file is recreated, then a call to \fBftok()\fR with the same
\fIpath\fR and \fIid\fR is likely to return a different key.
.sp
.LP
Only the low order 8-bits of \fBid\fR are significant.  The behavior of
\fBftok()\fR is unspecified if these bits are 0.
.SH RETURN VALUES
Upon successful completion, \fBftok()\fR returns a key.  Otherwise,
\fBftok()\fR returns \fB(key_t)\(mi1\fR and sets \fBerrno\fR to indicate the
error.
.SH ERRORS
The \fBftok()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEACCES\fR\fR
.ad
.RS 16n
Search permission is denied for a component of the path prefix.
.RE

.sp
.ne 2
.na
\fB\fBELOOP\fR\fR
.ad
.RS 16n
Too many symbolic links were encountered in resolving \fIpath\fR.
.RE

.sp
.ne 2
.na
\fB\fBENAMETOOLONG\fR\fR
.ad
.RS 16n
The length of the \fIpath\fR argument exceeds \fB{PATH_MAX}\fR or
a pathname component is longer than \fB{NAME_MAX}\fR.
.RE

.sp
.ne 2
.na
\fB\fBENOENT\fR\fR
.ad
.RS 16n
A component of \fIpath\fR does not name an existing file or \fIpath\fR is an
empty string.
.RE

.sp
.ne 2
.na
\fB\fBENOTDIR\fR\fR
.ad
.RS 16n
A component of the path prefix is not a directory.
.RE

.sp
.LP
The \fBftok()\fR function may fail if:
.sp
.ne 2
.na
\fB\fBENAMETOOLONG\fR\fR
.ad
.RS 16n
Pathname resolution of a symbolic link produced an intermediate result whose
length exceeds \fB{PATH_MAX}\fR.
.RE

.SH USAGE
For maximum portability, \fIid\fR should be a single-byte character.
.sp
.LP
Another way to compose keys is to include the project ID in the most
significant byte and to use the remaining portion as a sequence number. There
are many other ways to form keys, but it is necessary for each system to define
standards for forming them. If some standard is not adhered to, it will be
possible for unrelated processes to unintentionally interfere with each other's
operation. It is still possible to interfere intentionally. Therefore, it is
strongly suggested that the most significant byte of a key in some sense refer
to a project so that keys do not conflict across a given system.
.SH NOTES
Since the \fBftok()\fR function returns a value based on the \fIid\fR given and
the file serial number of the file named by \fIpath\fR in a type that is no
longer large enough to hold all file serial numbers, it may return the same key
for paths naming different files on large filesystems.
.SH ATTRIBUTES
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
.BR msgget (2),
.BR semget (2),
.BR shmget (2),
.BR stat (2),
.BR attributes (7),
.BR standards (7)
